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Funding Information: This work was supported by the Academy of Finland (Grant Nos. 317918, 325480, and 338748). Lithography and FIB milling were performed at the OtaNano-Micronova Nanofabrication Center and the OtaNano-NanoMicroscopy Center, respectively, supported by Aalto University. Computational resources were provided by the Aalto Science-IT project. We thank Helmholtz-Zentrum Berlin for the allocation of synchrotron radiation beamtime. We report the imaging of short-wavelength spin waves in a continuous nanometer-thick YIG film with a Co stripe patterned on top. Dynamic dipolar coupling between the YIG film and the Co stripe lowers the spin-wave wavelength when spin waves enter the YIG/Co bilayer region from the bare YIG film, causing partial reflection at the YIG/Co edge. We use time-resolved scanning transmission x-ray microscopy to image the mode conversion process down to a wavelength of 280 nm and extract the spin-wave dispersion, decay length, and magnetic damping in the YIG/Co bilayer. We also analyze spin-wave reflection from the YIG/Co edge and its dependence on the wavelength of incoming and transmitted spin waves.

Funding Information: Maxime Vergès was supported by a CIFRE Ph.D. grant from Vinci Technologies. This work was supported by the ANR‐20‐CE09‐0013 UFO, by the Institute Carnot ICEEL for the project “CAPMAT” and FASTNESS, by the Région Grand Est, by the Metropole Grand Nancy, for the Chaire PLUS by the impact project LUE‐N4S, part of the French PIA project “Lorraine Université d'Excellence” reference ANR‐15‐IDEX‐04‐LUE, by the “FEDERFSE Lorraine et Massif Vosges 2014–2020”, a European Union Program, and by the Academy of Finland (grant no. 316857). The devices in the present study were patterned at MiNaLor platform that is partially funded by Grand Est Region and Feder through RaNGE project. Publisher Copyright: © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH. The impact of plasmonic surface lattice resonances on the magneto-optical properties and energy absorption efficiency has been studied in arrays of [Co/Gd/Pt]N multilayer nanodisks. Varying the light wavelength, the disk diameter, and the period of the array, it is demonstrated that surface lattice resonances allow all-optical single pulse switching of [Co/Gd/Pt]N nanodisk arrays with an energy 400% smaller than the energy needed to switch a continuous [Co/Gd/Pt]N film. Moreover, the magneto-optical Faraday effect is enhanced at the resonance condition by up to 5,000%. The influence of the disk diameter and array period on the amplitude, width and position of the surface lattice resonances is in qualitative agreement with theoretical calculations and opens the way to designing magnetic metasurfaces for all-optical magnetization switching applications.

The impact of plasmonic surface lattice resonances on the magneto-optical properties and energy absorption efficiency has been studied in arrays of [Co/Gd/Pt]

Unlike classic synthetic stimulus-responsive and shape-memory materials, which remain limited to fixed responses, the responses of living systems dynamically adapt based on the repetition, intensity, and history of stimuli. Such plasticity is ubiquitous in biology, which is profoundly linked to memory and learning. Concepts thereof are searched for rudimentary forms of "intelligent materials." Here, we show plasticity of electroconductivity in soft ferromagnetic nickel colloidal supraparticles with spiny surfaces, assembling/disassembling to granular conducting micropillars between two electrodes driven by magnetic field

Funding Information: This work was supported by the Academy of Finland (Grants No. 295269, No. 306978, and No. 327804). K.R.E. acknowledges funding from the NSF (Grant No. NSF-MPS-2006456). E.G. was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo -FAPESP (Grant No. 18/19586-9). T.A.-N. has been supported in part by the Academy of Finland QTF CoE Grant No. 312298. Publisher Copyright: © 2022 American Physical Society. Magnetic skyrmions are topologically distinct particles whose thermally activated motion could be used to implement probabilistic computing paradigms. While solid-liquid phase transitions in skyrmion lattices have been demonstrated, the behavior of a skyrmion liquid and the effects of pinning are largely unknown. Here we demonstrate the formation of a weakly pinned skyrmion liquid in a magnetic heterostructure. By inserting a Ru wedge layer at the ferromagnet/heavy metal interface we evaluate the dependence of skyrmion dynamics on the skyrmion size and density. Our experiments demonstrate that the diffusion of skyrmions is largest in dense liquids with small skyrmions. The thermal motion of skyrmions at room temperature easily overcomes the narrow distribution of pinning site energies in the granular film structure, satisfying a key requirement of probabilistic device architectures. Micromagnetic simulations support the findings and also reveal the existence of a thermally activated high-frequency collective oscillation.

openaire: EC/H2020/861145/EU//BeMAGIC Funding Information: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 861145. The work was supported by the Academy of Finland (Grant Nos. 317918 and 325480), JST CREST under Grant No. JPMJCR18J1, and JSPS KAKENHI under Grant No. 21H04614. Lithography was performed at the OtaNano—Micronova Nanofabrication Centre of Aalto University. Computational resources were provided by the Aalto Science-IT project. Publisher Copyright: © 2022 Author(s). We report zero-field routing of spin waves in a multiferroic heterostructure comprising a ferromagnetic Fe film and a ferroelectric BaTiO3 substrate with fully correlated strain-coupled domains. In the Fe film, a regular alternation of magnetic anisotropy produces a back-and-forth rotation of uniform magnetization in zero magnetic field. Spin waves propagating across this domain structure are refracted at the magnetic domain walls because of abrupt changes in the dispersion relation and phase velocity. Using super-Nyquist sampling magneto-optical Kerr effect microscopy, we image the routing of spin waves and analyze the dependence of the effect on frequency and the propagation direction. We find that spin waves are routed efficiently by angles up to 60° without measurable loss in amplitude. The experimental results are reproduced by micromagnetic simulations and calculations based on the modified Snell's law for magnonics.

Funding Information: This work was supported by the Academy of Finland (Grant Nos. 295269, 306978, 321983, 325480, and 327804). We acknowledge the provision of computational resources provided by the Aalto Science-IT project. Publisher Copyright: © 2022 Author(s). Active control of propagating short-wavelength spin waves in perpendicularly magnetized materials is promising for designing nanoscale magnonic devices. One method of manipulating spin waves on the nanoscale is through their interaction with magnetic textures, an example of which is the magnetic skyrmion - a particle-like topological object stabilized in thin film heterostructures by the Dzyaloshinskii-Moriya interaction (DMI) and perpendicular magnetic anisotropy. In this paper, the interaction between spin waves and skyrmions is studied using micromagnetic simulations. The magnetic parameters chosen are similar to those found experimentally, leading to a skyrmion with an extended core of reversed magnetization. The effect of a propagating spin wave on the skyrmion is to cause the emission of a secondary spin wave by the skyrmion. At low frequencies, where the incoming spin wave wavelength is much larger than the skyrmion, this leads to a nearly circular re-emitted spin wave. The pattern of emission becomes increasingly complex at higher frequencies as the wavelength becomes similar to the skyrmion size due to the complex excitation of the extended core. The emitted spin wave profile can be controlled by altering the size of the skyrmion through the magnitude of the DMI, providing a method of tuning the system.

Spin waves are studied intensively for their intriguing properties and potential use in future technology platforms for the transfer and processing of information and microwave signals. The development of devices and materials for spin-wave systems requires a lot of measurement time and effort, and thus increasing the measurement throughput by extending the instrumentation capabilities is of the essence. In this letter, we report on a new and straightforward approach to increase the measurement throughput by fully exploiting the wideband detection nature of the Brillouin light scattering technique in single-shot experiments using a white-noise RF generator., Comment: 5 pages + 3 supplementary pages, 4 figures, 3 supplementary figures

We report experimental results on spin-wave propagation, transmission gap tuning, and mode conversion in straight, curved, and Y-shaped yttrium iron garnet waveguides with magnonic crystals made of submicrometer-wide airgrooves. We observe forbidden frequency gaps with sizes up to 200 MHz in straight waveguides and narrowing of the gaps in curved and Y-shaped waveguides. The spin-wave transmission signal is strongly suppressed inside the gaps and remains high at allowed frequencies for all waveguide types. Using super-Nyquist sampling magneto-optical Kerr effect microscopy, we image symmetric and asymmetric spin-wave interference patterns, the self-focusing of propagating spin waves, and interconversions between width modes with different quantization numbers.

Magnetic skyrmions are topologically-distinct swirls of magnetic moments which display particle-like behaviour, including the ability to undergo thermally-driven diffusion. In this paper we study the thermally activated motion of arrays of skyrmions using temperature dependent micromagnetic simulations where the skyrmions form spontaneously. In particular, we study the interaction of skyrmions with grain boundaries, which are a typical feature of sputtered ultrathin films used in experimental devices. We find the interactions lead to two distinct regimes. For longer lag times the grains lead to a reduction in the diffusion coefficient, which is strongest for grain sizes similar to the skyrmion diameter. At shorter lag times the presence of grains enhances the effective diffusion coefficient due to the gyrotropic motion of the skyrmions induced by their interactions with grain boundaries. For grain sizes significantly larger than the skyrmion diameter clustering of the skyrmions occurs in grains with lower magnetic anisotropy., 7 pages, 5 figures

The possibility of tuning the magnetic properties of materials with voltage (converse magnetoelectricity) or generating electric voltage with magnetic fields (direct magnetoelectricity) has opened new avenues in a large variety of technological fields, ranging from information technologies to healthcare devices and including a great number of multifunctional integrated systems, such as mechanical antennas, magnetometers, and radio frequency (RF) tunable inductors, which have been realized due to the strong strain-mediated magnetoelectric (ME) coupling found in ME composites. The development of single-phase multiferroic materials (which exhibit simultaneous ferroelectric and ferromagnetic or antiferromagnetic orders), multiferroic heterostructures, as well as progress in other ME mechanisms, such as electrostatic surface charging or magneto-ionics (voltage-driven ion migration), have a large potential to boost energy efficiency in spintronics and magnetic actuators. This article focuses on existing ME materials and devices and reviews the state of the art in their performance. The most recent progress on different ME devices based on ME heterostructures is presented but with a larger emphasis on ME antennas and sensors due to the significant advances achieved in these applications. The rapid development of mechanically actuated ME antennas has been observed over the past several years, producing ME antennas that are miniaturized by 1-2 orders compared to conventional antenna size. Magnetic sensors based on simple ME composites are potentially promising alternatives to conventional magnetometers due to their very good detectivity (, The work of Patrick Hayes, Viktor Schell, Eckhard Quandt, and Jeffrey McCord was supported by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) through the Collaborative Research Center CRC 1261 "Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics." The work of Pedro Martins was supported in part by the Fundacao para a Ciencia e Tecnologia (FCT) in the framework of the Strategic Funding under Grant UID/FIS/04650/2020 and under Project PTDC/BTMMAT/28237/2017 and Project PTDC/EMD-EMD/28159/2017, in part by the Fundacao para a Ciencia e Tecnologia (FCT) for the contract under the Stimulus of Scientific Employment, Individual Support-2017 Call, under Grant CEECIND/03975/2017, in part by the Spanish State Research Agency (AEI), in part by the European Regional Development Fund (ERFD) under Project PID2019-106099RB-C43/AEI/10.13039/501100011033, and in part by the Basque Government Industry and Education Department under the ELKARTEK, HAZITEK, and PIBA (Grant PIBA-2018-06) Programs. The work of Marisa Medarde was supported in part by the Swiss National Science Foundation under Grant 200021-141334 and Grant 206021_139082, in part by the Swiss National Center of Competence in Research MARVEL (Computational Design and Discovery of Novel Materials) under Grant 1NF40_182892, in part by the European Community's 7th Framework Program under Grant 290605 (COFUND:PSI-FELLOW). The work of Senentxu Lanceros-Mendez was supported in part by the Fundacao para a Ciencia e Tecnologia (FCT) in the framework of the Strategic Funding under Grant UID/FIS/04650/2020 and under Project PTDC/BTM-MAT/28237/2017 and Project PTDC/EMD-EMD/28159/2017, in part by the Spanish State Research Agency (AEI), in part by the European Regional Development Fund (ERFD) under Project PID2019-106099RB-C43/AEI/10.13039/501100011033, and in part by the Basque Government Industry and Education Department under the ELKARTEK, HAZITEK, and PIBA (Grant PIBA-2018-06) Programs. The work of Sebastiaan van Dijken was supported

We introduce and explore low-energy configurations in two-dimensional arrays consisting of Ising-type dipolar coupled nanomagnets lithographically defined onto three-nanomagnet vertices arranged in a triangular coordination. Thus, the system is dubbed the trimerized triangular lattice. Employing synchrotron-based photoemission electron microscopy, we perform temperature-dependent magnetic imaging of moment configurations. These states are then characterized in terms of spin correlations and magnetic structure factors. The results reveal a competition between ferromagnetic and vortex dominated orders, which can be controlled by varying the relevant lattice parameter and the corresponding competing interactions.

openaire: EC/H2020/745115/EU//OPLD Organic light-emitting diodes (OLEDs) have been established as versatile light sources that allow for easy integration in large-area surfaces and flexible substrates. In addition, the low fabrication cost of OLEDs renders them particularly attractive as general lighting sources. Current methods for the fabrication of white-light OLEDs rely on the combination of multiple organic emitters and/or the incorporation of multiple cavity modes in a thick active medium. These architectures introduce formidable challenges in both device design and performance improvements, namely, the decrease of efficiency with increasing brightness (efficiency roll-off) and short operational lifetime. Here we demonstrate, for the first time, white-light generation in an OLED consisting of a sub-100 nm thick blue single-emissive layer coupled to the photonic Bragg modes of a dielectric distributed Bragg reflector (DBR). We show that the Bragg modes, although primarily located inside the DBR stack, can significantly overlap with the emissive layer, thus efficiently enhancing emission and outcoupling of photons at selected wavelengths across the entire visible light spectrum. Moreover, we show that color temperature can be tuned by the DBR parameters, offering great versatility in the optimization of white-light emission spectra.

Magnetoelectric coupling in multiferroic heterostructures offers a promising platform for electric-field control of magnonic devices based on low-power spin-wave transport. Here, electric-field manipulation of the amplitude and phase of propagating spin waves in a ferromagnetic Fe film on top of a ferroelectric BaTiO3 substrate is demonstrated experimentally. Electric-field effects in this composite material system are mediated by strain coupling between alternating ferroelectric stripe domains with in-plane and perpendicular polarization and fully correlated magnetic anisotropy domains with differing spin-wave transport properties. The propagation of spin waves across the strain-induced magnetic anisotropy domains of the Fe film is directly imaged and it is shown how reversible electric-field-driven motion of ferroelectric domain walls and pinned anisotropy boundaries turns the spin-wave signal on and off. Furthermore, linear electric-field tuning of the spin-wave phase by altering the width of strain-coupled stripe domains is demonstrated. The results provide a new route toward energy-efficient reconfigurable magnonics.

openaire: EC/H2020/948260 /EU//PLAS-OLED The nanoscale mode volumes of surface plasmon polaritons have enabled plasmonic lasers and condensates with ultrafast operation1–4. Most plasmonic lasers are based on noble metals, rendering the optical mode structure inert to external fields. Here we demonstrate active magnetic-field control over lasing in a periodic array of Co/Pt multilayer nanodots immersed in an IR-140 dye solution. We exploit the magnetic nature of the nanoparticles combined with mode tailoring to control the lasing action. Under circularly polarized excitation, angle-resolved photoluminescence measurements reveal a transition between the lasing action and non-lasing emission as the nanodot magnetization is reversed. Our results introduce magnetization as a means of externally controlling plasmonic nanolasers, complementary to modulation by excitation5, gain medium6,7 or substrate8. Further, the results show how the effects of magnetization on light that are inherently weak can be observed in the lasing regime, inspiring studies of topological photonics9–11.

Voltage manipulation of skyrmions is a promising path towards low-energy spintronic devices. Here, voltage effects on skyrmions in a GdOx/Gd/Co/Pt heterostructure are observed experimentally. The results show that the skyrmion density can be both enhanced and depleted by the application of an electric field, along with the ability, at certain magnetic fields to completely switch the skyrmion state on and off. Further, a zero magnetic field skyrmion state can be stablized under a negative bias voltage using a defined voltage and magnetic field sequence. The voltage effects measured here occur on a few-second timescale, suggesting an origin in voltage-controlled magnetic anisotropy rather than ionic effects. By investigating the skyrmion nucleation rate as a function of temperature, we extract the energy barrier to skyrmion nucleation in our sample. Further, micromagnetic simulations are used to explore the effect of changing the anisotropy and Dzyaloshinskii-Moriya interaction on skyrmion density. Our work demonstrates the control of skyrmions by voltages, showing functionalities desirable for commercial devices., Comment: 5 pages, 5 figures

We present a realization of highly frustrated planar triangular antiferromagnetism achieved in a quasi-three-dimensional artificial spin system consisting of monodomain Ising-type nanomagnets lithographically arranged onto a deep-etched silicon substrate. We demonstrate how the three-dimensional spin architecture results in the first direct observation of long-range ordered planar triangular antiferromagnetism, in addition to a highly disordered phase with short-range correlations, once competing interactions are perfectly tuned. Our work demonstrates how escaping two-dimensional restrictions can lead to new types of magnetically frustrated metamaterials.

The ability to imprint a given material property to another through a proximity effect in layered 2D materials has opened the way to the creation of designer materials. Here, molecular-beam epitaxy is used for direct synthesis of a superconductor-ferromagnet heterostructure by combining superconducting niobium diselenide (NbSe

Funding Information: This work was supported by the Academy of Finland (Grant Nos. 316857 and 295269). The authors wish to acknowledge CSC – IT Center for Science, Finland, and the Aalto Science‐IT project for computational resources. Publisher Copyright: © 2022 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH. The voltage control of magnetism is a promising path to the development of low-power spintronic devices. Magneto-ionics—exploiting voltage-driven ion migration to control magnetism—has attracted interest because it can generate large magnetoelectric effects at low voltage. Here, the use of the solid-state lithium-ion battery technology for reversible voltage-controlled switching between perpendicular and in-plane magnetization states in a Co–Pt bilayer is demonstrated. Due to the small size and high mobility of lithium ions, small voltages produce an exceptionally high magnetoelectric coupling efficiency of at least 7700 fJ V–1 m–1 at room temperature. The magnetic switching effect is attributed to the modulation of spin-orbit coupling at the Co–Pt interface when lithium ions migrate between a lithium storage layer (LiCoO2) and the magnetic interface across a lithium phosphorous oxynitride (LiPON) solid-state electrolyte, which is corroborated by density functional theory calculations. Voltage control of magnetism in the battery structure does not show degradation over more than 500 voltage cycles, demonstrating promise for solid-state lithium-based magneto-ionic devices.

Active control over the concentration or distribution of oxygen vacancies in transition metal oxides enables manipulation of their structural, magnetic, electronic transport, and optical properties. Topotactic oxidation or reduction reactions involving annealing under different ambient conditions are often used to manipulate the oxygen concentration. In this paper, using in situ scanning transmission electron microscopy, the authors demonstrate a reversible structural phase transition in perovskite La${}_{0.5}$Sr${}_{0.5}$CoO${}_{3\ensuremath{-}\ensuremath{\delta}}$ films during heating/cooling cycles while the environment and oxygen vacancy concentration are kept constant. Switching between two oxygen-deficient structures is shown to arise from a local reordering of the oxygen vacancies by thermal strain imposed by the substrate. This approach presents new opportunities for switchable ionic devices.

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The Dzyaloshinskii-Moriya interaction gives rise to a chiral exchange between neighboring spins in the technologically relevant class of perpendicularly magnetized ultrathin film materials. In this paper, we study the temperature dependence of the Dzyaloshinskii-Moriya interaction based on extensive characterization of a thin film which hosts a skyrmion state using both bulk magnetometry and x-ray magnetic circular dichroism photoemission electron microscopy. A version of the Bloch law explicitly for thin film geometries is derived to extract the exchange stiffness. The strength of the Dzyaloshinskii-Moriya interaction, $D$, is found to have a dependence on the saturation magnetization, ${M}_{s}$ of $D\ensuremath{\propto}{M}_{s}^{1.86\ifmmode\pm\else\textpm\fi{}0.16}$. Further, by extracting the uniaxial anisotropy ${K}_{u}$ and the exchange stiffness $A$, we find that $D\ensuremath{\propto}{K}_{u}^{1.02\ifmmode\pm\else\textpm\fi{}0.11}$ and $D\ensuremath{\propto}{A}^{0.95\ifmmode\pm\else\textpm\fi{}0.07}$. Skyrmion radii are also used to extract the strength of the Dzyaloshinskii-Moriya interaction which is compared to that derived from measurements of stripe domains. The origins of the correlations between material parameters are discussed and consequences of these relationships for skyrmion devices are considered.

We demonstrate a tenfold resonant enhancement of magneto-optical effects in perpendicularly magnetized ${[\mathrm{Co}/\mathrm{Pt}]}_{N}$ nanodots mediated by the excitation of optimized plasmon modes. Two magnetoplasmonic systems are considered: square arrays of ${[\mathrm{Co}/\mathrm{Pt}]}_{N}$ nanodots on glass and identical arrays on a $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_{2}$ bilayer. On glass, the optical and magneto-optical spectra of the nanodot arrays are dominated by the excitation of a surface lattice resonance (SLR), whereas on $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_{2}$, a narrow surface plasmon polariton (SPP) resonance tailors the spectra further. Both the SLR and SPP modes are magneto-optically active leading to an enhancement of the Kerr angle. We detail the dependence of optical and magneto-optical spectra on the number of Co/Pt bilayer repetitions, the nanodot diameter, and the array period, offering design rules on how to maximize and spectrally tune the magneto-optical response of perpendicularly magnetized ${[\mathrm{Co}/\mathrm{Pt}]}_{N}$ nanodots.

Deterministic polarization reversal in ferroelectric and multiferroic films is critical for their exploitation in nanoelectronic devices. While ferroelectricity has been studied for nearly a century, major discrepancies in the reported values of coercive fields and saturation polarization persist in literature for many materials. This raises questions about the atomic‐scale mechanisms behind polarization reversal. Unconventional ferroelectric switching in ε‐Fe2O3 films, a material that combines ferrimagnetism and ferroelectricity at room temperature, is reported. High‐resolution in situ scanning transmission electron microscopy experiments and first‐principles calculations demonstrate that polarization reversal in ε‐Fe2O3 occurs around pre‐existing domain walls only, triggering local domain wall motion in moderate electric fields of 250–500 kV cm−1. Calculations indicate that the activation barrier for switching at domain walls is nearly a quarter of that corresponding to the most likely transition paths inside ε‐Fe2O3 domains. Moreover, domain walls provide symmetry lowering of the polar structure near the domain boundary, which is shown to be necessary for ferroelectric switching in ε‐Fe2O3. Local polarization reversal in ε‐Fe2O3 limits the macroscopic ferroelectric response and offers important hints on how to tailor ferroelectric properties by domain structure design in other relevant ferroelectric materials., This work was supported by the Academy of Finland (Grant Nos. 293929, 304291, 319218 and 316857) and by the European Research Council (ERC‐2012‐StG 307502 and ERC‐2018‐CoG 819623). STEM analysis was conducted at the Aalto University OtaNano‐Nanomicroscopy Center (Aalto‐NMC). ICMAB acknowledges financial support from the Spanish Ministry of Economy, Competitiveness and Universities, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV‐ 2015‐0496) and the project MAT2017‐85232‐R co‐financed with the EU FEDER program, as well as the Generalitat de Catalunya (projects 2017SGR1377 and 2017SGR765). The authors thank Jaume Gàzquez for discussions about the work and critical reading of the manuscript. K.Z.R. and M.L. gratefully acknowledge the financial support by Deutsche Forschungsgemeinschaft (DFG) through DFG‐ANR GALIMEO project (LE 2504/2‐1), as well as the support of Jülich Supercomputing Centre (JSC, project jiff38) and JARA‐HPC Partition (projects jara0081 and jara0126). R.C.Y. and X.X.G. acknowledge the financial support by the National Key Research Program of China (Grant No. 2017YFA0206200) and the National Natural Science Foundation of China (Grant No. 11874413). We thank Prof. Nathalie Viart for valuable discussions.